function out = doubleFilter(filter)
%the program double the filter by inserting zeros between neighbouring
%filter bank coefficients.
%parameter : 
%filter : the input filter . Type : cell.
%output :
%out : the output doubled filter. Type : cell
nfilters = numel(filter);
H = cell(1,nfilters);
len = length(filter{1});
for k = 1 : nfilters
    H{k} = zeros(1,2*len-1);
    H{k}(1:2:2*len-1) = filter{k};
end
out = H;
end
